1. Field of the Invention
The invention relates to the standardization of tester systems, in particular tester systems having a test card for making contact with integrated circuits on a wafer.
2. Description of the Related Art
When integrated components are manufactured on a wafer, their function is also tested before the wafer is sawn up. The test is typically done by fitting a test card, which usually has contact devices in the form of needles arranged in accordance with the contact faces of the integrated circuits, on the wafer in order to make contact with the contact faces. The contact making devices are connected by means of tester channels to a tester system which transmits tester signals to the integrated circuits and receives tester signals from the integrated circuits.
In order to test the integrated circuits at a high frequency, the tester channels must be standardized such that the time reference and the signal strength of the test signals which are received by the integrated circuits correspond as precisely as possible to the test signals which are generated by the tester system, and the test signals which are transmitted by the integrated circuits correspond as precisely as possible to the test signals which are received by the tester system. In order to standardize such a tester system, the tester channels are each connected to an adjustable delay unit and an adjustable signal amplifier which are set individually during the standardization process in such a way that signal transit time differences and attenuation differences on the tester channels can be compensated by setting the individual delay in the delay elements and the amplification by the signal amplifier. If the tester channels are embodied as individual line connections between the tester system and the integrated circuit, that is to say, as what is referred to as a single transmission line, the tester channel can be standardized by measuring and evaluating the voltage profile and time profile of the reflections of a test signal when the tester channel is not connected.
When two lines are used per tester channel in what is referred to as dual transmission line systems, this method of standardization is no longer possible since the lines are terminated in an adapted fashion and thus no reflection occurs at the open terminal. The dual transmission lines disconnect the lines for the signal running to the integrated circuit and the signal running back from the integrated circuit to the tester system.
In order to standardize the tester channels, a standardization robot is usually used which individually moves along the contacts of a base board and makes contact with a contact tip in order to connect them to measuring lines of the test system. The tester channels are successively adjusted in this way with respect to the signal strength and the signal transit time.
However, currently, such standardization robots can only be used for standardizing tester channels which are connected to base boards for accommodating housed integrated circuits since on such boards the dimensions of the contact points with which contact is to be made are sufficiently large and are at a sufficient distance from one another in order to have contact made with them using the contact tip.
When dual transmission line systems are used for testing integrated circuits which are located still in the unsawn state on the wafer, such a standardization robot cannot be used since the contact making devices which are located on the test card have excessively small dimensions and are too short a distance from one another in order to be able to reliably access them with the contact tip of the standardization robot. Furthermore, in the tester systems for testing integrated circuits, the test cards are permanently mounted on a wafer and are not accessible from the outside. For this reason also, the standardization robot cannot be used for standardizing the tester channels.
Even in tester systems which are to test at high frequencies and which use only single test lines as tester channels, the precision of the reflection standardization is not sufficient.
The previous procedure of testing the tester channels of a test system using a test card takes the form of additionally implementing a standardization pad in the tester device. This standardization pad can be positioned electromechanically and can be moved by means of the control of the test card to the contact making device which is to be standardized. The mechanism makes contact between the standardization pad and the contact making device of the test card. By means of this contact, the test system is connected to a reference comparator or to a reference driver in order to carry out the standardization. One disadvantage of this system is that the standardization pad has to be successively positioned on the contact making device at the tester channel to be standardized, which is time consuming and increases the risk of damage to the contact making devices.
Therefore, there is a need to provide for the standardization of a tester system which may be easily and reliably standardized using a test card for testing integrated circuits on an unsawn wafer.